Connection components with posts

ABSTRACT

A subtractively created interconnection scheme and apparatus, typically used with microelectronic devices, wherein a flexible support structure is attached to a conductive sheet. The conductive sheet is then selectively removed, preferably using an etching process, thereby producing a plurality of posts with tips which are substantially coplanar with respect to one another. Each post becoming an individual interconnection between the microelectronic device and a supporting substrate.

CROSS-REFERENCE TO RELATED APPLICATIONS

[0001] The present application is a continuation of U.S. patentapplication Ser. No. 09/707,452, which application is a divisional ofco-pending U.S. patent application Ser. No. 08/885,238, which issued asU.S. Pat. No. 6,177,636 on Jan. 23, 2001 and which is a continuation ofU.S. patent application Ser. No. 08/366,236 filed on Dec. 29, 1994.

FIELD OF THE INVENTION

[0002] The present invention relates, generally, to interconnectingmicroelectronic devices and supporting substrates, and more particularlyrelates to an apparatus and a method of interconnecting microelectronicdevices to supporting substrates using subtractively created members.

BACKGROUND OF THE INVENTION

[0003] Complex microelectronic devices such as modem semiconductor chipsrequire many hundreds of input and output connections to otherelectronic components. These device connections are generally eitherdisposed in regular grid-like patterns, substantially covering thebottom surface of the device (commonly referred to as an “area array”)or in elongated rows extending parallel to and adjacent each edge of thedevice's front surface. The various prior art processes for making theinterconnections between the microelectronic device and the supportingsubstrate use prefabricated arrays or rows of leads/discrete wires,solder bumps or combinations of both, such as with wire bonding, tapeautomated bonding (“TAB”) and flip/chip bonding.

[0004] In a wirebonding process, the microelectronic device may bephysically mounted on a supporting substrate. A fine wire is fed througha bonding tool and the tool is brought into engagement with a contactpad on the device so as to bond the wire to the contact pad. The tool isthen moved to a connection point of the circuit on the substrate, sothat a small piece of wire is dispensed and formed into a lead, andconnected to the substrate. This process is repeated for every contacton the chip. The wire bonding process is also commonly used to connectthe die bond pads to lead frame fingers which are then connected to thesupporting substrate.

[0005] In a tape automated bonding (“TAB”) process, a dielectricsupporting tape, such as a thin foil of polyimide is provided with ahole slightly larger than the microelectronic device. An array ofmetallic leads is provided on one surface of the dielectric film. Theseleads extend inwardly from around the hole towards the edges of thehole. Each lead has an innermost end projecting inwardly, beyond theedge of the hole. The innermost ends of the leads are arranged side byside at a spacing corresponding to the spacing of the contacts on thedevice. The dielectric film is juxtaposed with the device so that thehole is aligned with the device and so that the innermost ends of theleads will extend over the front or contact bearing surface on thedevice. The innermost ends of the leads are then bonded to the contactsof the device, typically using ultrasonic or thermocompression bonding,and the outer ends of the leads are connected to external circuitry.

[0006] In both wire bonding and conventional tape automated bonding, thepads on the substrate are arranged outside of the area covered by thechip, so that the wires or leads fan out from the chip to thesurrounding pads. The area covered by the entire assembly isconsiderably larger than the area covered by the chip. This makes theentire assembly substantially larger than it otherwise would be. Becausethe speed with which a microelectronic assembly can operate is inverselyrelated to its size, this presents a serious drawback. Moreover, thewire bonding and tape automated bonding approaches are generally mostworkable with chips having contacts disposed in rows extending along theedges of the chip. They generally do not allow use with chips havingcontacts disposed in an area array.

[0007] In the flip-chip mounting technique, the front or contact bearingsurface of the microelectronic device faces towards the substrate. Eachcontact on the device is joined by a solder bond to the correspondingcontact pad on the supporting substrate, as by positioning solder ballson the substrate or device, juxtaposing the device with the substrate inthe front-face-down orientation and momentarily reflowing the solder.The flip-chip technique yields a compact assembly, which occupies anarea of the substrate no larger than the area of the chip itself.However, flip-chip assemblies suffer from significant problems whenencountering thermal stress. The solder bonds between the devicecontacts and the supporting substrate are substantially rigid. Changesin the relative sizes of the device and the supporting substrate due tothermal expansion and contraction in service create substantial stressesin these rigid bonds, which in turn can lead to fatigue failure of thebonds. Moreover, it is difficult to test the chip before attaching it tothe substrate, and hence difficult to maintain the required outgoingquality level in the finished assembly, particularly where the assemblyincludes numerous chips.

[0008] As the number of interconnections per microelectronic deviceincreases, the issue of interconnection planarity continues to grow aswell. If the interconnections are not planar with respect to each other,it is likely that many of the interconnections will not electricallycontact their juxtaposed contact pads on a supporting substrate, such asa standard printed wiring board. None of the above described techniquesprovides a cost effective interconnection scheme which guarantees theplanarity of the interconnections so that each is assured of making anelectrical contact with the contact pads on the opposed supportingsubstrate.

[0009] Numerous attempts have been made to solve the foregoinginterconnection problems. An interconnection solution put forth in U.S.Pat. No. 4,642,889, entitled “Compliant Interconnection and MethodTherefor” issued Apr. 29, 1985 to Grabbe creates an interconnectionscheme by embedding wires within each solder column/ball to reinforcethe solder thereby allowing higher solder pedestals and more elasticity.Further interconnection solutions put forth include providing acombination of solder and high lead solder thereby allowing highersolder pedestals and more elasticity given the high lead content of thesolder, as found in U.S. Pat. No. 5,316,788, entitled “Applying Solderto High Density Substrates” issued May 31, 1994 to Dibble et al. andU.S. Pat. Nos. 5,203,075 & 5,133,495, respectively issued on Apr. 20,1993 and Jul. 28, 1992 to Angulas et al.

[0010] U.S. Pat. No. 4,955,523, entitled “Interconnection of ElectronicComponents” issued on Sep. 11, 1990 to Calomragno et al. puts forth astill further interconnection technique in which wires are wirebonded tothe contact pads on a first surface, cut to a desired length and thenattached to a second opposing surface by placing each of the wires in a“well” of conductive material, such as solder. While the wires then givea certain amount of compliancy to the structure, this techniqueencounters difficulties in controlling unwanted bending and electricalshorting of the wires prior to and during the coupling step in theirrespective solder wells. Similarly, U.S. Pat. No. 5,067,007, entitled“Semiconductor Device having Leads for Mounting to a Surface of aPrinted Circuit Board” issued Nov. 19, 1991 to Kanji et al. disclosesthe use of stiff or deformable lead pins to increase the pin pitch anddeal with problems stemming from thermal coefficient of expansionmismatches between the device and a printed circuit board. Besides thepotential for bending and shorting of the pins as described above, thepins are individually attached to both the device and the printedcircuit board by brazing or soldering making this a time consuming andless than optimum solution from a manufacturing point of view.

[0011] U.S. Pat. No. 4,067,104, entitled “Method of Fabricating an Arrayof Flexible Metallic Interconnects for Coupling MicroelectronicComponents” issued on Jan. 10, 1978 to Tracy uses an additive techniquewhere the interconnections are created by providing a layer ofphotoresist, removing portions of the photoresist and depositing metalwithin the removed portions. By successively following this technique, aplurality of metalized columns are created and coupled to opposingcontact pads on a supporting substrate by a suitable method, such asflip chip bonding, cold welding, diffusion bonding or melting. Thephotoresist is then removed. However, interconnection planarity issuescan become a problem when practicing the invention disclosed in Tracy'104. Further, the strength of each of the interconnection columns maybe impeded due to the joining of the different layers of metal and tothermal cycling fatigue.

[0012] One commonly assigned invention, U.S. patent application Ser. No.08/190,779, filed Feb. 1, 1994 and issued as U.S. Pat. No. 5,445,390 onOct. 3, 1995, deals effectively, but specifically differently, with manyof the problems encountered by the prior art. In one embodiment, the'779 application interconnects the device contact pads to the supportingsubstrate terminals by using leads which are coupled to the terminals ina conventional manner, such as by soldering, such that they extendsubstantially side by side. The unconnected ends are then coupled to thecontact pads through the use of predetermined pressure and temperatureconditions. A support layer is then disposed between the device and thesupporting substrate and further surrounds and supports the leads. Thisstructure effectively deals with thermal expansion mismatch and leadshorting problems.

[0013] Despite these and other efforts in the art, still furtherimprovements in microelectronic interconnection technology would bedesirable.

SUMMARY OF THE INVENTION

[0014] The present invention provides a method and apparatus forproviding interconnections between a microelectronic device and asupporting substrate which substantially obviates many of the problemsencountered by the prior art.

[0015] One embodiment of the present invention provides a method offabricating an interconnection component for a microelectronic devicecomprises providing a support structure, typically comprised of aflexible but substantially inextensible substrate, having a first and asecond surface, where a conductive sheet is coupled to the first surfaceof the support structure. The conductive sheet is then selectivelyremoved, typically using an etching process, thereby producing a highlyplanar, cost effective plurality of substantially rigid posts each ofwhich eventually become the interconnections between the microelectronicdevice and a supporting substrate. The etching process generally firstincludes applying a photoresist layer to the conductive sheet andexposing portions of the photoresist layer to form etch resistantportions and remainder portions. The remainder portions may then beremoved and the conductive sheet may be etched around the etch resistantportions.

[0016] A compliant layer may then be provided on the second surface ofthe support structure and a microelectronic device having a plurality ofbond pads may be engaged with the exposed surface of the compliantlayer. The compliant layer is used to substantially accommodate thermalcoefficient of expansion mismatches between the device and a supportingsubstrate when the device is in use. Each bond pad is then electricallycoupled to at least one conductive post. The bond pads and posts may becoupled in a number of different ways, including plating a plurality ofetch resistant conductive leads on either the first surface of thesupport structure or the conductive sheet such that the leads aresandwiched between the supporting substrate and the conductive sheet.After the posts are created, the bond pads may be electrically connectedto respective leads. Alternately, the conductive leads could be formedon the second surface of the support structure and coupled to each postthrough a conductive via. A highly conductive layer, such as gold, mayoptionally be plated on the surface of the posts to ensure a goodelectrical connection when the posts are coupled to contact pads on asupporting substrate.

[0017] The foregoing and other objects and advantages of the presentinvention will be better understood from the following DetailedDescription of a Preferred Embodiment, taken together with the attachedFigures.

BRIEF DESCRIPTION OF THE DRAWINGS

[0018]FIGS. 1A and 1B are perspective views illustrating the process ofsubtractively creating the interconnections according to one embodimentof the present invention.

[0019]FIGS. 2A and 2B are perspective views each showing the embodimentin FIG. 1B coupled to a compliant layer and a microelectronic deviceaccording to one embodiment of the present invention.

[0020] FIGS. 3A-C are perspective views each showing one possible shapeof the subtractively created interconnections according to the presentinvention.

[0021]FIG. 4A is an elevational view of a post and socket.

[0022]FIG. 4B is a perspective view of a post and socket.

[0023]FIG. 5 is a perspective view of a subtractively createdinterconnection having an etch resistant, conductive cap thereonaccording to the present invention.

[0024]FIG. 6A is an elevational view of a post.

[0025]FIG. 6B is a top view of a brazing button and hole.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT

[0026] Referring to FIGS. 1A and 1B, a top surface of a supportstructure 100 is coupled to a conductive sheet 110. In the preferredembodiment of the invention, the support structure is a flexible, butsubstantially inextensible, film preferably formed from a polymericmaterial, such as Kapton™, of an approximate thickness between 25microns and 75 microns and is laminated to the second surface of theconductive sheet. However, the support structure could be comprised ofmany other suitable materials and may further be semi-flexible orsubstantially rigid. The conductive sheet 110 is preferably comprised ofa conductive metal, such as copper, copper alloys or phosphor bronze,among other materials. Portions of the conductive sheet are selectivelyremoved by any suitable means to create a plurality of subtractivelycreated, substantially rigid posts 130, as shown in FIG. 1B.

[0027] In the preferred embodiment, portions of the conductive sheet areremoved by first providing a photoresist mask on the surface of theconductive sheet and etching away the conductive sheet 110 around themask portions. This is preferably accomplished by coupling a photoresistlayer 120 to the top surface of the conductive sheet 110. Selectedportions of the photoresist layer 120 are then exposed and developedusing standard industry techniques, resulting in a plurality of etchresistant photoresistive portions 125 atop the conductive sheet 100. Aone sided etching process is then employed to remove portions of theconductive sheet 110 around the plurality of etch resistantphotoresistive portions 125 while substantially leaving the portionsbeneath the plurality of etch resistant photoresistive portions 125, asshown in FIG. 1B. The etch profile of features created from a conductivesheet, such as a metal foil, can be influenced by the process used toproduce them. The two most common methods of etching are batch immersionin an etchant solution and liquid etchant spraying or impingement. Inbatch etching, the features can be more uniformly created. Etchingproceeds isotropically removing metal at a basically uniform rate bothvertically and laterally. This results in creating posts havingsubstantially uniformly sloping vertical sides of approximately a 45°angle relative to the surface of the support structure. Etching normallyproceeds rather slowly in batch processing providing sufficient time toreplenish the active etchant solution to foil under the resist. Incontrast, a spray etching technique typically impinges the part at moreof a 90° angle, facilitating the etching of surfaces exposed to theimpingement. While the etching process still progresses in a more orless isotropic fashion, the etch resistant photoresist portions 125 actas a shield causing the etching process to produce an etch profile whichforms “cooling tower” shaped posts 130 having a broad base which thinsas it reaches the vertical center of the post 130 and flares back outslightly as it reaches its apex. These features are caused by the“splash back” of the etchant solution against the walls of the emergingpost and can be more or less exaggerated by altering the pressure,concentration and or formula of the etchant within the bounds of thephotoresist's resistance to the etchant.

[0028] The height of each post will vary directly with the thickness ofthe conductive sheet 110, but typically will be in the range of 125 to500 microns. Because of their shape and rigidity, the conductive posts130 will resist deformation. A fine post connect pitch can therefore becreated without substantial fear that the posts 130 will be bent intoelectrical contact with each other. The possible pitch of the bumps isalso a function of the thickness of the sheet of conductive material.The thinner the conductive sheet, the finer the possible pitch of thebumps. Also, this process of creating the posts is cost and timeeffective when compared with methods which create each bump by platingor soldering. Further, the posts created with this subtractive processare extremely uniform and planar when compared to solder or plated bumpsbecause they are created from a single planar, conductive sheet. Thisensures that each of the bumps will make contact with respective contactpads on a supporting substrate, such as a printed wiring board, withoutthe exertion of undue pressure on the top surface of the microelectronicdevice.

[0029] The exterior surfaces of the posts may be optionally plated witha highly conductive layer, such as gold, gold/nickel, gold/osmium orgold/palladium, or alternately plated with a wear resistant, conductivecoating such as osmium to ensure that a good connection is made when theposts are either soldered or socketed to a supporting substrate, asdescribed more fully below.

[0030] Referring now to FIG. 2A, a compliant layer 140 is coupled to theback surface of the support structure 100. The compliant layer 140 istypically made of an elastomer material, such as the Dow Coming siliconelastomer 577 known as Silgard®. The compliant layer 140 is coupled tothe back surface of the support structure 100 by conventional stencilprinting techniques. The silicon elastomer used in the preferredembodiment is filled with about 5-10% of fumed silica in order to obtaina stiff consistency that allows the layer 140 to retain its shape afterthe stencil is removed. The silicon is then cured at a suitabletemperature. Typically, the thickness of the complaint layer is 150microns, plus or minus 12.5 microns. The compliant layer 140 mayalternately be replaced with a plurality of compliant pads 145 eachpositioned beneath a respective post, as shown in FIG. 2B. The pads 145are also typically stenciled on the back surface of the supportstructure 100 and the original stiff formulation of the elastomer allowseach individual pad 145 to retain its shape after the stencil has beenremoved. The exposed surface of the compliant layer is next engaged witha surface of a microelectronic device 150 having a plurality of bondpads 160 thereon.

[0031] Referring now to FIG. 2B, before the bond pads 160 can beconnected to the conductive posts 130, a method of electricallyconnecting the posts 130 to the bond pads 160 must be supplied. Onemethod includes providing etch-resistant conductive leads 170, such ascopper leads which have been lithographically formed on the top surfaceof the support structure 100 plated with gold prior to coupling thestructure 100 to the conductive sheet 110. After the conductive sheet110 has been reduced to the conductive posts 130, shown in FIG. 2A, theetch resistant conductive leads may be connected to the bond pads 160 byany suitable manner, such as wire bonding or by allowing the leads toextend beyond the edge of the support structure such that they may bebent towards and thermosonically or ultrasonically bonded to arespective bond pad, as shown in FIG. 2A. An alternate method ofcreating a similar embodiment is to first plate a plurality of eitherone layer or a multi-layer etch resistant conductive leads, such as goldor gold/copper leads, to the bottom surface of the conductive sheet 110prior to coupling the conductive sheet 110 and the support structure100. Portions of the conductive sheet are then removed to create theconductive posts 130 allowing the bond pads 160 to be electricallyconnected to the posts 130 by the conductive leads. A further alternatesolution involves forming the leads on the second side of the supportstructure 100 and connecting the posts through conductive vias extendingfrom the first to the second surface of the support structure 100.

[0032] A further embodiment of the present invention, includes directlyattaching the support structure 100 to the microelectronic device suchthat each post is in electrical contact with a juxtaposed bond pad onthe microelectronic device. This is typically accomplished using aconductive via positioned beneath each of the posts and extending from afirst to a second surface of the support structure. The via may becreated by punching or laser ablating holes in the support structure andplating a conductive metal, such as copper into each of the holes. Ajoining layer, such as a gold/tin or silver/tin alloy, is next typicallycoupled to the copper. The joining layer will weld to its respectivebond pad under the correct temperature, pressure or vibration stresses.

[0033] As stated above, the shape of the posts 130 can depend on theprocess used to remove the surrounding conductive material. However, theshape of the etch resistant photoresist portions 125 in FIG. 1A may alsoproduce different shaped posts from the conductive sheet material. Forexample, FIG. 3A shows a substantially in the form of a surface ofrevolution which is the result of using circular resist portions 180 onthe conductive sheet 110. Square resist portions 190 will produce a posthaving four slightly concave, rounded sides meeting at slightly roundededges, as shown in FIG. 3B. Triangular resist portions 200 will producea post having three slightly concave, rounded sides meeting at slightlyrounded edges, as shown in FIG. 3C. Each of these photoresist portionsproduce the “cooling tower” shape shown if a spray etching process isused. If a batch immersion process is used, the resulting posts willhave more linearly sloping vertical walls and slightly sharper comers.

[0034] The peaks of the posts 130 may then be coupled to the contactpads on the supporting substrate by any suitable means, such as directlysoldering the posts to the contact pads or inserting them into socketsattached to the substrate. The “cooling tower” shape created by sprayetching makes for a more reliable leaf-spring socket connection becauseits peak has a larger diameter than its middle section, as shown in FIG.4A. The peak of the post will thus provide resistance to being pulledout of the socket in response to forces acting in the lengthwise planeof the posts. The vertical comers on the posts shown in FIGS. 3B and 3Cpartially inserted into round socket holes or vias also makes for a morereliable, force fit, separable, electrical connection with each sockethole contact, as shown in FIG. 4B.

[0035]FIG. 5 shows a further embodiment in which the photoresist layer120, in FIG. 1, is replaced with a plurality of metallic portions 210 ofa geometry similar to the photoresist portions (180/190/200) in FIGS.3A-C. Typically, the metallic portions 210 are comprised of an etchresistant metal, such as nickel. The conductive layer may then be etchedaround the metallic portions 210 leaving the post capped with aconductive top. This conductive top may then be plated with a highlyconductive layer, such as gold or a gold alloy. This conductive topfurther increases the reliability of an electrical connection when theposts are inserted into the type of socket shown in FIG. 4A. In analternate embodiment, solder can also be used as an etch resist. Afterthe posts are created, the solder can then be reflowed to create asolder coated post. If the solder is reflowed after the post has beeninserted into a test socket, it will create a more permanent electricalconnection with the socket.

[0036] FIGS. 6A-B show a still further embodiment having a brazingbutton 220 extending through brazing hole in a removable supportstructure 230. The brazing button is used to attach the post directly toa bond pad on a microelectronic device and is typically comprised of ametallic alloy which will attach easily and provide a good electricalconnection with its respective bond pad, such alloys include gold-tin,bismuth-tin, gold-silicon, or tin-silver. FIG. 6B shows one embodimentof a brazing hole 240 which allows for expansion of the brazing buttonwhen it is heated to attach to the chip bond pad. The removable supportstructure 230 is comprised of a material which may be removed by anysuitable means after the posts have been attached to the bond pads, suchas using a paper or water soluble polymeric support structure which maybe sprayed with water and peeled off.

[0037] One skilled in the art will appreciate that the subtractivelycreated posts described herein could be used for many other purposesbesides connecting microelectronic devices to supporting substrateswithout departing from the spirit of the present invention. Further, ifthe top surfaces of the posts are sufficiently wide, a cupped portioncould be provided thereon to receive bumps or solder balls on thesurface of a supporting substrate.

[0038] Having fully described several embodiments of the presentinvention, it will be apparent to those of ordinary skill in the artthat numerous alternatives and equivalents exist which do not departfrom the invention set forth above. It is therefore to be understoodthat the present invention is not to be limited by the foregoingdescription, but only by the appended claims.

1. A component comprising: (a) a conductive layer including a plurality of conductive metallic posts extending substantially parallel to one another, said posts having tips; and (b) a bonding material carried on said posts, said bonding material covering the tips of said posts but not covering other portions of said posts
 2. A component as claimed in claim 1 wherein said bonding material is a solder.
 3. A component as claimed in claim 1 wherein said posts are elongated and said bonding material projects outwardly from said posts at said tips in directions transverse to the direction of elongation of said posts.
 4. A component as claimed in claim 1 wherein said bonding material covers only the tips of said posts.
 5. A component as claimed in claim 1 wherein said bonding material is in the form of individual portions, each such portion overlying the tip of one of said posts.
 6. A component as claimed in claim 1 wherein said portions are circular discs and said posts have surfaces substantially in the form of surfaces of revolution coaxial with said discs.
 7. A component as claimed in claim 1 wherein each said post has a height of 125 to 500 microns.
 8. A component as claimed in claim 1 further comprising a support structure, said posts projecting from said support structure so that said tips are remote from said support structure, said posts having bases adjacent said support structure.
 9. A component as claimed in claim 8 wherein said posts are tapered so that the base of each post is wider than the tip of such post.
 10. A component as claimed in claim 8 wherein said support structure includes a polymeric layer.
 11. A component as claimed in claim 10 wherein said support structure includes conductive leads on said polymeric layer connected to said posts.
 12. A component as claimed in claim 11 wherein said posts project from a first surface of said polymeric layer and said conductive leads are disposed on said first surface of said polymeric layer and the bases of at least some of said posts are connected to said conductive leads. 